This invention relates to an exhaust gas disposable catalyst having a superior property of decomposing NOx and a method of manufacturing the same.
In order to follow fuel restrictions for a mobile engine, it is planned to employ a lean-burn engine. Further, it should be prevented as possible to exhaust NOx in gas exhausted from the engine to the air.
In some proposed methods for preventing the NOx gas exhaustion, in case with a mobile engine, it is practiced to decompose NOx by providing a catalyst converter at an exhaust pipe of the engine. Insufficient decomposition of NOx is obtained by a conventional three way catalyst, however, in case of high concentration of oxygen in the exhaust gas.
As a catalyst for effectively decomposing NOx, there is a known zeolite catalyst supporting copper by eon-exchange and a .gamma.-alumina catalyst. Another technique has been disclosed in Japanese Patent Application Laying Open Gazette No.1-139144 that a catalyst composed of a mixture of zeolite supporting a transition element by ion-exchange and alumina supporting a catalyst element is used for decomposing NOx.
In spite of the high-level decomposing rate (catalytic conversion efficiency) of over 90% in a laboratory, however, in an actual mobile engine, the catalyst for decomposing the above mentioned NOx has a problem that the catalyst cannot decompose NOx effectively because:
a) the temperature of an actual exhaust gas of an automobile does not necessarily accord with a temperature of the catalyst capable of decomposing Nox (activation temperature);
b) a composition of the actual exhaust gas of the automobile is delicately different from that at a rig test in a laboratory; and
c) an actual exhaust gas flow of the automobile is faster than that at a rig test in a laboratory.
The temperature of the actual exhaust gas shall rises high to 800.degree.-900.degree. C. according to an operational condition of the engine, which causes an insufficient heat resistance with the zeolite as a catalyst supporter.
Moreover, in a .gamma.-alumina catalyst in which .gamma.-alumina supports an element having a catalysis, the element of the catalyst is merely, physically supported by the .gamma.-alumina. Hence, the catalyst element is attrited owing to an external force such as vibration.
The present invention has its object to provide a catalyst capable of effectively decomposing the exhaust gas of a mobile engine which is actually used. In detail, the object is to provide an exhaust gas disposable catalyst which has high durability under the above mentioned high exhaust gas temperature and excellent NOx decomposing power under conditions at low exhaust gas temperature and with lean mixture.
Another object of the present invention is to provide a method of manufacturing the catalyst having the above superior property.